1. Field of the Invention
The present invention relates generally to electrical transmission and interconnection systems for plural load circuit control and particularly to such systems employing sequential selection of the controlled load circuits. The present invention relates particularly to apparatus for the control of electrical power demand in a plurality of loads so as to reduce the total system power requirements to a pre-selected maximum level.
Electrical utility systems have been designed to operate as nearly as possible in an optimal fashion while responding to customers' instantaneous demands for electrical power. In recent years, however, during periods of high electrical power demand, "brown-outs" and even "black-outs" have been a common experience, an experience which experts believe will become even more common in the absence of some widespread adoption of consumer load management. These brown outs and black outs which occur during emergency conditions are achieved by various relay schemes (under-frequency, under-voltage, over-current, etc.) resistor switching, and other power system stabilizing techniques which usually result in some form of load shedding. Typically, these load shedding schemes include voltage reduction, area black outs, volt power substation load curtailment, and disconnecting interruptable customers. Unfortunately, all but the last of these strategies disconnect loads in a gross fashion without recognizing the significance of individual loads which often results in customer dissatisfaction with utility performance and occasionally results in system-wide failure.
The efficiency of power production for a utility depends to a large extent on the uniformity and predictability of the level of power required to meet customer demand. In order to improve system efficiency, utilities have adopted billing practices designed to encourage customers to even out their power consumption and to avoid temporary over-loads that are incapable of being efficiently met by the power sources available to the utility. One practice is to charge the customer according to a usage formula that includes a factor based on the maximum power consumption in each of a succession of time intervals usually referred to as demand intervals. Utility meters have been developed for installation at the power entry point of the customer's facility for measuring both the total energy consumed during a billing period and the maximum energy used during the demand intervals.
The customer is thereafter billed a disproportionately higher amount for power consumed during the high demand intervals thereby encouraging the customer to regulate or manage his power consumption. This customer level load management also permits an overall reduction in power consumption since some uses for the power, once postponed, will be eliminated or at least diminished. Customer level load management still provides for essential services to the customer and generally provides for higher system-wide reliability, thereby increasing customer satisfaction with utility performance.
2. Description of the Prior Art
Various customer level load management schemes have been proposed based on a cycling or deferment of loads from periods of high power demand to periods of lower power demand. Load shedding then occurs in response to a signal indicating that electrical power usage has exceeded defined permissible limits. Unfortunately, such systems generally have a tendency to shed maximum loads towards the end of demand periods frequently causing relatively rapid change of loads which is not only annoying to the customer but also objectionable to the power utility as it causes periodic power surges in the lines. Commonly such systems make no provision to limit the number of stages or loads on or off and as a result the customer does not take advantage of the potential savings inherent in peak shaving at all times rather than just during periods of maximum power consumption.
Other customer level load management systems found in the prior art employ motor driven rotary switches, solenoid-actuated stepping switches, or equivalent solid state devices to connect a series of customer loads in a certain sequence and, after a certain period of time or upon sensing of a certain external condition, disconnecting the customer loads in the same or opposite sequence. The sequence is then repeated cyclically so as to maintain the load at a desired load level condition. While load management of this fashion is generally preferred over the line condition responsive systems previously reviewed, a common failure of most systems is the inability to change the sequence in response to varying external environmental conditions and the absence of any means for avoiding start-up transients in the event of system-wide power failure.
Examples of the prior art are to be found in U.S. Pat. Nos. 3,205,368; 3,489,913; 3,659,114; 3,714,453; 3,984,699; and 4,031,406.